1. Field of the Invention
The invention relates to pulsed light sources, more especially, but not exclusively, to light sources that comprise optical fiber amplifiers for chirped pulse amplification.
2. Related Art
There is much demand for sources of pulses of light having both ultrashort durations (picosecond and femtosecond durations) and high energies. Many applications of ultrashort optical pulses require the peak powers, and hence the energies, of the pulses to be sufficient to excite a wide variety of non-linear optical processes. These include two-photon absorption, non-linear frequency generation in optical parametric oscillators, and super continuum generation in non-linear small core optical fibers. For materials processing, there is a rapidly emerging demand for femtosecond and picosecond pulses at ultraviolet wavelengths (which can be generated by frequency-quadrupling processes). It is observed that unlike ultraviolet pulses having nanosecond durations (produced by Q-switching processes), ultrashort pulses can ablate material before heat has time to diffuse away from the exposed region. This results in improved resolution and reduced collateral damage such as micro-cracking. Furthermore, given sufficiently high beam quality and amplitude stability, it is also possible to detect (by two-photon absorption) and ablate (in materials processing) features of a dimension less than the wavelength of the light, by arranging that only in the bright center of the beam is the intensity high enough to excite the required non-linear processes. Meeting the requirements for this wide range of existing and emerging applications therefore requires pulses at a variety of wavelengths with preferably sub-picosecond duration and megawatt peak power, so that the pulse energies are in the microjoule region.
Silica fiber doped with ytterbium ions (Yb3+) is a useful medium for the generation and amplification of ultrashort pulses at around 1 micron, and more specifically at 1.06±0.05 μm. Yb3+-doped fiber has a broad gain bandwidth, high optical conversion efficiency and a large saturation fluence [1]. Several ultrashort pulse systems using Yb3+-doped fiber have been reported. Examples include:                a laser having an Yb3+-doped fiber gain medium in a ring-shaped cavity and pumped with a Ti-sapphire laser [1]. The repetition rate of the pulses generated is ˜50 MHz;        a system including a laser having an Er-fiber gain medium producing 1.56 μm picosecond pulses that are successively put through a fiber Raman shifter, a periodically poled lithium niobate (PPLN) second harmonic generator and a Yb3+-doped fiber pre-amplifier to convert the pulses to a wavelength of 1.055 μm [2], at a repetition rate of 50 MHz; and        a laser using a Yb3+-doped fiber gain medium to generate femtosecond pulses at a repetition rate of 50 MHz, in which pump light is coupled into the cladding of the fiber [3].        
Yb3+-doped fiber lasers will generally have too low an output energy for many of the non-linear applications detailed above. Laser oscillators tend to be unstable when operated so as to produce suitably high output powers directly. Consequently, optical amplification, also by Yb3+-doped fiber, has been proposed for the amplification of pulses having wavelengths around 1 μm [4,5].
The use of fiber amplifiers as amplification stages for pulsed laser sources presents its own problems, principally mode distortion, non-linear distortion and gain saturation.
Mode distortion arises if multimode fibers are used to increase energy storage [6]. Single mode fibers have good mode characteristics, but cannot provide such high energy storage. An amplifier using a multimode fiber in which only the fundamental propagation mode is excited has been proposed [7] to combine high energy storage and good mode characteristics.
Non-linear distortion in the amplifier gain medium arises at high peak pulse powers which result when higher overall energies are desired. A technique for reducing non-linear distortion is chirped pulse amplification (CPA) in which pulses are stretched (or chirped) prior to amplification and then compressed (or unchirped) after amplification. Pulse stretching increases pulse durations during amplification, thereby lowering peak pulse powers below the threshold at which non-linear effects are induced in the gain medium of the amplifier.
Gain saturation arises at high pulse repetition rates, such as the 50MHz repetition rates of the systems described above, where the average power of the pulses becomes so high that the gain of the amplifier saturates. This reduces the pulse energies which can be achieved by the amplification process.
There is thus the need for a high-energy pulsed Yb3+laser source at a wavelength of around 1 micron that offers a good combination of low mode distortion and low non-linear distortion while avoiding gain saturation.